Retractable pellet impact drill bit



Oct. 19, 1954 J. M. CAMP ETAL RETRACTABLE PELLET IMPACT DRILL BIT Filed April 23, 1952 2 Sheets-Sheet 1 6 pm 6 b a Q. v 0 A n n a. Q Q I I ma QM o m @E a A5 on aw UN 5 5 I GW I J I 7 .4 fiA m f w Oct. 19, 1954 J, M. CAMP ET AL 2,692,116

RETRACTABLE PELLET IMPACT DRILL BIT Filed April 23, 1952 2 Sheets-Sheet 2 H- 1- I; O1

331 53 Envenbovs Patented Oct. 19, 1954 RETRACTABLE PELLET KMPACT DRZLL BIT John IV. Camp and John E. Eckel, Tulsa, Okla assignors to Standard Oil Development Company, a corporation of Delaware Application April 23, 1952, Serial No. 283,836

7 Claims.

This invention concerns a novel form of drill bit of a nature permitting insertion and retraction of the drill bit through and from a drill string or casing maintained in drilling position in a borehole. The drill bit of this invention embodies what has been called the pellet impact drilling method. In accordance with this method, apparatus is provided to eject a high velocity directed jet of fluid in which a multitude of spherical pellets are entrained. Impact of these pellets against the formation to be drilled causes a fracturing or pulverizing action to accomplish effective drilling. It is the purpose of this inven tion to provide a drill bit arrangement such that the full benefits of pellet impact drilling may be obtained while permitting insertion and retraction of the drill bit through a drill string on which the drill bit is to be employed.

The present invention couples two basic objectives to provide a desirable form of drill bit.

First the invention utilizes the impact of a multitude of pellets to secure drilling action. It has been found that this type of drilling offers many advantages. For example, pellet impact drilling does not require maintenance of Weight on the drill bit so that lightweight tubing may be employed if desired. Again rotation of the drill bit is optional and if employed may utilize a slow rotational rate requiring little rotary torque. Finally, drill bits of the character permitting pellet impact drilling may be constructed to have a prolonged operational life, as no portion of the drill bit is called on to directly bear against the formation for drilling purposes.

The objective of employing this desirable form of drilling is coupled in this invention with the objective of permitting drilling with casing. In drilling with casing, the purpose is to support the drilling tool on a string of conventional casing rather than on the drill pipe which is conventionally employed. The casing will have a diameter somewhat less than the borehole drilled so that it can be lowered without obstruction into the borehole. To make this technique practical it is necessary that all drilling operations be conducted without necessity for withdrawing the casing from the hole. This entails the necessity for providing an extensible form of drill bit which can be passed through the casing and extended below the casing to drill a borehole which is greater in diameter than the casing. Again it is necessary that such a drill bit be retractable to permit removal through the casing itself without necessitating removal of the casing from the borehole.

As stated therefore, it is the purpose of the present invention to provide a retractable drill bit employing the pellet impact principle which may be employed in a drilling with casing operation.

The basic principles of pellet impact drilling are fundamentally simple. Means are simply required to provide a jet of high velocity fluid including provision for entraining and accelerating pellets in this jet of fluid. The requisites of this drilling procedure concern the nature of the pellets employed, and the fluid employed therewith as a propelling and recirculation agent for the pellets.

The pellets to be employed must be of substantial size. Thus it has been found that granular pellets or pellets having a diameter less than about A; of an inch are relatively ineffective in drilling. In generalthe rate of drilling attainable increases as the size of the pellets is increased. For this reason it is generally preferred to employ the largest pellets possible, consistent with the nozzle size of the apparatus and the fracturing characteristics or" the pellets. As will be emphasized, the nature of the drilling fluid employed also has a bearing on the size of the pellets. However, it may be stated that when employing a gas, such as air, for the drilling fluid, pellets of about to a quarter of an inch in diameter are to be employed. When using a liquid, such as water or drilling mud, as the drilling fluid, the pellets may range in size from about A; to 1 inch. The preferred size range in this case is between about and A of an inch.

It is important that the pellets have the greatest practical density. It is apparent that the greater the density of a given sized pellet the greater the kinetic energy which can be attained and the greater the impact force which can be developed. 'Again, as will be brought out, the separation characteristics of the pellets from the drilling fluid employed play an important part in drilling eficiency. Since separation rate of the pellets from a fluid depends in part upon the density or" the pellets, it is apparent that this fac tor again dictates use of a high density pellet.

The configuration and surface characteristics or" the pellets are also important. The pellets should be substantially spherical in nature and should have a smooth surface so as to limit wear of the jet nozzle assembly through which the pellets are to be ejected. In this connection it is fundamental that a spherical body possesses the best resistance to fracture due to impact; no other shape possesses the mechanical strength of a sphere. The factors of spherical configura tion and smooth surface also relate to the separation characteristics of the pellets from the entraining fluid as will be noted below.

In View of these considerations, the pellets to be employed are generally metal spheres having the desired properties of impact resistance, hardness, and toughness. Iron, steel and other ferrous alloys may be employed to prepare the pellets.

In this connection, however, it should be observed that it is essential that the metal chosen be nonbrittle in character. Due to their high density, tungsten carbide alloys of the less brittle character are attractive for use in the pellets to be employed. Again, alloys of this character or other dense metals may be employed as a pellet cor material, surfaced by ferrous alloys having the requisite toughness.

As suggested heretofore, the separation characteristics of the pellets from the drilling fluid play an important part in the efficiency of pellet impact drilling. It is apparent that in jetting a stream of fluid, entraining the pellets referred to, against a formation to be drilled, it becomes important that the pellets readily separate from this stream of fluid. This is required so that the, pellets will not be cushioned by the fluid and will not be inefliciently swept from the drilling zone in the fluid flow stream. Again, regardless of the type of pellet recirculation employed, it becomes necessary in some manner to subsequently secure separation of pellets from an upward flowof drilling mud in the borehole for return and recirculation of the pellets to the drilling zone.

Consequently, as emphasized heretofore, pellets of the character defined must be employed to have the separation characteristics required for effective drilling. In this connection it is apparent that the nature of the drilling fluid employed also affects the separation characteristics of the pellets. From this viewpoint it is desirable to employ a drilling fluid having the lowest viscosity and lowest density practical, consistent with other requirements of the drilling fluid. It has been found in the work heretofore completed that the drilling rate obtainable is proportional to the ratio of the density of the pellets to the density of the drilling fluid. This consideration makes the use of air or other gas attractive. Similarly, if a liquid is to be employed, water or other low density liquid is preferably employed. Nonetheless, if considered desirable, drilling muds of the character conventionally used in rotary drilling may be employed.

Recirculation of the pellets in the immediate vicinity of the drilling zone may be accomplished in a number of ways. When the pellets have been forced against a formation'by jetted fluid, the fluid flow will carry the pellets outwardly and upwardly in an annular channel along the wall of the borehole. At some point spaced above the bottom of the borehole, it becomes necessary therefore to separate the entrained pellets from the upflowing drilling fluid. One method by which this may be accomplished is to provide a low velocity fluid zone above the drilling apparatus in which the pellets may settle by gravity into a recirculation system. Such a low velocity zone may be provided by narrowing the cross-sectional area of the apparatus immediately above the drill so as to provide an enlarged annular space in the borehole. This gravity recirculation can be supplemented or substituted by a mechanical deflection system serving to maintain the pellets in captive recirculation. A shroud may be employed for this purpose of a character substantially blocking the borehole but contain ing passages through which drilling fluid and pulverized earth formation may be carried to the surface of the earth.

It is generally desirabl in obtaining a suitable ejection of pellets to employ a nozzle assembly utilizing a primary and secondary jet nozzle. The primary nozzle is employed to convert fluid pumping pressure to velocity energy. Consequently fluid pumped through the primary nozzle is subjected to a substantial pressure drop, of pounds per square inch or more, to provide a constricted directed high velocity jet of fluid. A secondary nozzle is positioned immediately below and concentric withthe primary nozzle. The secondary nozzle should have a substantially greater diameter than the primary nozzle to accommodate the total volume of fluid from the primary nozzle in addition to the volume of pellets there entrained. This configuration and arrangement of nozzles, by its nature, creates an aspirating effect which plays a part in the recirculation of the pellets.

In accordance with this invention a pellet impact drill bit is provided embodying the basic principles of pellet impact drilling heretofore described. In the preferred form of the drill bit the drill itself comprises a primary and secondary nozzle through which fluid and pellets may be entrained and forcefully directed against the bottom of a borehole. Preferably the drill bit includes a shroud element of a slotted nature which encircles the nozzle assembly to permit continuous captive recirculation of the drilling pellets employed. The elements of this drill bit are segmented and provided as separate elements attachable to supporting arrangements permitting the elements to be sequentially lowered to and raised from drilling position through the drill string or casing employed as the tubular support member.

Although it is possible to drill a bore hole with a pellet drill having a shroud and a fixed position secondary nozzle constructed as a single rigid piece, it is necessary to remove the drill pipe or casing in order to replace such a single piece assembly when it becomes worn excessively by pellet impact. The segmented shroud of the present invention permits the drill pipe (or casing) to remain in the bore hole and the shroud and nozzles to be withdrawn independently for replacement.

The retractable drill bit of this invention utilizes a tubular drill collar in which a cylindrical mandrel element is slidably positioned. The mandrel element supports the nozzle assembly for jetting of fluid and pellets at the lower termination thereof. A number of longitudinal slots or grooves are cut in the mandrel element corresponding to the number of segments of the shroud employed. At least three segments and three grooves must be employed although it is preferred to employ four to provide a shroud arrangement which is segmented into four sections. A sliding support member for each of the shroud segments is arranged for longitudinal movement in the grooves of the mandrel within the limits of the drill collar. The support member associated with each segment may slide with respect to the mandrel so as to maintain the different portions of the shroud at different elevations when in the retracted position. Each shroud segment is p-ivotally attached to its support member so that in retracted position the segments may swing inwardly within the diameter of the tubing employed. A suitable arrangement of latching dogs is employed to control the extension and retraction of the elements: of the drill and to secure the sliding positioning of the support members referred to.

A preferred embodiment of the drill bit of this invention is illustrated in the accompanying drawings. In thesedrawings a retractable drill bit is illustrated which employs a shroud element which is divided into four segments to permit passage through the casing employed.

Figures 1 and la of the drawings taken together, of which Figure 1a is the lower extension of Figure 1, illustrate the drill of this invention in elevational cross-sectional detail, showing the drill bit in an extended and looked drilling position within a borehole.

Figure 2 is a cross-sectional view of the app-aratus of Figure 1 along the line II--II, showing the arrangement of the sliding support members in the slots of the mandrel element. Figure 2 particularly shows the line along which the sectional view of Figure 1 is taken.

Figure 3 is a cross-sectional view of Figure 1 along the line III-III of Figure 1a, particularly showing the means of supporting the secondary nozzle of the apparatus from the mandrel element and showing the slotted perforations employed at the upper portion of the shroud element.

Finally, Figure 4 is a cross-sectional elevational view of the lower portion of the drill bit showing the arrangement of the elements of the drill bit when in retracted position within the drill collar or casing employed.

Referring first to Figures 1 and 1a, the drill bit illustrated will be described to indicate the essential nature of the drill bit. The drill assembly illustrated is suspended in a borehole 2 by means of a tubular support member 1 which may constitute a large diameter drill pipe or casing. A restricted nozzle opening 4 is provided which will hereafter be referred to as. a primary nozzle element. The primary nozzle is in direct fluid communication with the interior of casing I. Consequently, drilling fluid pumped downwardly through casing i will be ejected from primary nozzle 4 as a high velocity jet of fluid. Directly below primary nozzle 5 is a secondary nozzle element 5 which is spaced from but coaxial with primary nozzle 4. The secondary nozzle has a substantially greater diameter than the primary nozzle, and serves to entrain and accelerate pellets in the fiuid jet passed through the secondary nozzle.

A multitude of pellets of the nature hereinbefore described may be continuously recirculated to secure drilling by simply employing the primary and secondary arrangements illustrated independent of the other elements of the drawing. Thus, assuming that pellets are entrained in the fluid ejected through the secondary nozzle from the primary nozzle, these pellets will forcefully impinge against the bottom of the borehole to cause a cutting pattern of the general nature illustrated. The pellets will be forced outwardly and upwardly by the flow of drilling fluid to a point somewhat above the upper lip of the secondary nozzle. Here, due to the substantial increase in size of the secondary nozzle compared to the primary nozzle, the pellets may be caught in a strong aspirating force so as to be pulled into the secondary nozzle for continued circulation and drilling.

In this type of drill bit, however, it is particularly preferred that means he provided to cause the captive recirculation of the pellets, eliminating any possibility for pellets to pass upwardly in the borehole to be lost from the drilling zone. For this purpose a shroud element 6 is preferably employed. As shown, shroud 6 is a sleeve element maintained in fixed position about the primary and secondary nozzles and extending at least some distance below the secondary nozzle. The lower edge of this shroud may be tapered as shown. The upper converging portion of the shroud adjacent its juncture with the primary nozzle element is. slotted as indicated by slots 7. These slots are constructed to have an opening which is smaller than the diameter of the pellets employed so as to prevent the passage of pellets therethrough.

Employing this shroud element operation of the drill is in the same general manner as that described. However, the perforated shroud ele ment prevents escape of pellets from the drilling zone and forces recirculation of the pellets into the secondary nozzle. Drilling fiuid and pulverized earth may however pass through the slotted perforations to pass upwardly in the annulus oi the borehole to the surface of the earth.

In accordance with this invention a drill bit operating in the manner described and comprising the essential elements identified is constructed so as to permit extension and retraction through the tubular supporting member or casing i. To achieve this the shroud element is divided into four segments as indicated in Figure 3. In retracting the drill bit, provision is made to fix two opposed segments in the position illustrated in Figure 1 while permitting inward pivoting and withdrawal of the other pair of segments through the casing. At a later operational time period the remaining pair of segmented shroud members may then be pivoted inwardly and supported below the first pair of shroud elements within the casing. The primary and secondary nozzle elements are positioned on a mandrel element which during retraction is first withdrawn and which during extension is employed to lock the shroud segments in fixed position.

The mandrel element referred to is identified by numeral 8, and simply comprises a tubular member fitting within the drill collar 3 in sliding telescopic arrangement. The lower termination of mandrel 3 is provided with a restriction which constitutes the primary nozzle i. The secondary nozzle 5 which may simply constitute a tubular member is supported from the mandrel element 8 below the primary nozzle by means of two or more supporting arms 9. The supporting arms 9 are positioned and constructed to obstruct recirculation of pellets to secondary nozzle 5 as little as practicable.

As particularly shown in Figure 2 a number of longitudinal slots or grooves are cut in the mandrel element 3. In the embodiment of the drill illustrated, four such longitudinal slots are employed which are spaced at around the circumference of the mandrel extending from an upper portion of the mandrel completely to the bottom thereof. A sliding support member for each of the four segments of the shroud is positioned in each of the slots of the mandrel. These sliding supporting members will hereafter be identified as guide members serving to maintain the proper positioning of the shroud segments at all times during operation and serving to guide the shroud segments to and from their extended positions. In Figures 1 and 1a, two of the guide members are illustrated, identified by numbers it and i I. As shown in these figures, each guide member extends downwardly within the groove provided for it in mandrel 8 to support a shroud segment at the lower termination of the guide member. At some distance above the shroud segment a pivot joint or hinged joint is provided. Preferably as illustrated in Figures la and 4, two

such articulated joints, l3 and 14, are employed. A latching dog i is pivotally fixed within a slot of each of the guide members. These latching dogs are normally urged outwardly by a spring element. Recesses into which these latching dogs can extend outwardly are provided at an appropriate elevation in the drill collar 3. For this purpose an annular groove can be machined in the drill collar 3 providing a shoulder against which each latching dog I5 can bear to prevent further downward movement of each guide member with respect to the drill collar 3. The latching dogs 15 in Figure 1 are illustrated in the extended position within the annular groove provided in the drill collar 3. The bed of each of the longitudinal grooves provided in mandrel 8 is formed to provide a retraction slot for each of the latching dogs I 5 associated with the guide member operating in the particular groove. Thus a retraction slot i6 is provided in the bed of the groove of mandrel 3 in which guide member I9 is positioned. Similarly a retraction slot ii is provided in the bed of the groove in which the guide member H is positioned.

Space limitations of the drawings prevent illustration of the critical positioning of the latches i5 with respect to the retraction slots referred to in each of the grooves. However, as will be better understood from the description of operation which follows, certain critical displacements of these elements must be maintained. Thus the latching dogs If: in the position illustrated in Figure 1 must be at a distance from the uppermost recess 16 greater than the distance between the primary nozzle 4 and the bottom of the secondary nozzle 5. The uppermost retraction slot it must be at a distance above the lower retraction slot ll greater than the overall length of each shroud segment.

The shroud retracting slots it and I'l cut in the grooves of the mandrel serve to control retraction of the difierent segments of the shroud in a desired sequence. If desired, the slots corresponding to H5 and IT in each of the four grooves of the mandrel may each be at a different level on the mandrel where the distance between each slot is greater than the overall length of the shroud element. Alternatively, and more simply, the slots provided in two diametrically opposed grooves may be at one and the same level while the slots provided in the other of the two diametrically opposed grooves may be at a second level. The displacement between these slots, as indicated, is greater than the overall length of the shroud element.

The general operation of the apparatus permitting extension and retraction may now be understood. Assuming that the apparatus is in drilling position as shown in Figures 1 and la the sequence of operations occurring during retraction will first be described. A suitable wireline tool is attached to latches 2B of mandrel 8 so as to permit pulling the mandrel upwardly through the drill collar 3 and the casing i. Latches 2B are pivotally fixed in slots of the mandrel 8 and are normally urged outwardly into the latching position shown by springs 2!. A spearhead tool having a lower cylindrical extension forcing retraction of the latches over the spearhead may be employed. Applying a lifting force to this tool, mandrel 8 will be pulled upwardly with respect to the other elements shown in the drawing. During the firstoperational step mandrel 8 is pulled upwardly while all other elements of the drill are maintained in locked position. This occurs by virtue of the arrangement of the latching dogs I5 associated with each of the guide members. Each of the latching dogs 15 is held in the extended position to lock each guide member against movement until the appropriate retraction slot provided on the mandrel registers with the latching dogs. Thus, mandrel 8 will be pulled upwardly until the primary and secondary nozzles are at a level adjacent pivot point E3 of guide member iii. At this stage in the operation retraction slots It will register with dog l5 of guide member H3. The frictional force of mandrel 8 with respect to guide member l9 aided by the tapered surface of the groove in the mandrel collar 3 will force dog l5 into recess I6. Continued upward movement of the mandrel 8 will thereafter raise the guide member and the dependent shroud segment upwardly into the drill collar. Pivoting of the guide member at pivots i3 and [4 will permit the segment of the shroud to swing inwardly within the drill collar. It may be observed that in the preferred embodiment of the invention a diametrically opposed guide member will at the same instant be raised upwardly in this same manner. However, upward movement of the other two guide members will be prevented until dog 55 associated with these guide members registers with the lower shroud retraction slot in mandrel 8. This will occur when mandrel 8 carrying the first pair of shroud segments is raised above the second pair of shroud segments. At this step in the operation latching dog l5 associated with guide member I! and its diametrically opposed guide member may seat in the appropriate retraction slot H to permit raising of the second pair of shroud segments in the same manner as that described. 7

The general configuration of these elements in the retracted position is illustrated in Figure 4 showing the manner in which sequential stacking of the different elements in the drill collar and casing is achieved. 7

In lowering the drill bit illustrated through a casing for extension to the position shown in Figures 1 and 1a, the reverse sequence of steps occurs. A simple spearhead is attached to latching dogs 29 so as to permit lowering of the apparatus. As the apparatus is lowered into the drill collar 3, latching dogs l5 associated with the lowermost pair of shroud segments will first seat in the latching recesses provided by the lower annular groove of drill collar 3 as shown in Figure 1. This will prevent further downward movement of the lowermost pair of shroud segments. Continued lowering of the drill apparatus will bring the latching dogs associated with the upper pair of shroud segments to latching position in drill collar 3. Again, further downward movement of this pair of the shroud segments will be prevented. At this step in the operation, the four shroud. segments will be substantially in the position illustrated in Figure 1a..

However, additional downward movement of the mandrel 8 permits dogs 2d to expand and thereby lock in the upper annular groove of drill collar 3, holding the assembly in drillingposition. When this occurs, latches 2%} will extend to release the spearhead of the lowering tool and mandrel 8 will be locked in position fixing all elements of the drill bit in a clamped position.

It is preferred to position a suitable sealing element between the mandrel element 8 and the drill collar 3 to prevent any bypassing of fluid therebetween. A suitable means for accomplishing this objective is diagrammaticallyillustrated employing the ports 25 and the elastic packer element 26. Packer 25 may constitute rubber or rubber impregnated fabric which is positioned around mandrel 8 as a circumferential band. A number of ports 25 may be cut through mandrel 8 to provide fluid access to the greater portion of the surface of packer 26 from within the drill apparatus. Consequently, when fluid is pumped through casing I for operation of the drill bit, fiuid will exert pressure through ports 25 to force the packer 26 against the drill collar to maintain these elements in sealed relationship.

The apparatus heretofore described may also be desirably modified by inclusion of limit stops on the mandrel and guide members to insure positive action of the elements during retraction of the shroud. For this purpose, an enlarged annular collar may be positioned on the lower portion of the mandrel providing a shoulder 32 as shown in Figure la. Mating shoulders 31 and 33 may be provided on each of the guide members. The distance between shoulder M of guide member Ill and shoulder 32 of mandrel 8 will be critically set so that these shoulders will bear on each other when mandrel 8 is moved upwardly with respect to guide member it so that latching dog 55 of guide member Hi registers with the retraction slot i6. Consequently continued upward movement of the mandrel will lift the guide member, positively forcing the dog I into its retraction slot. Shoulder 33 of guide member H is similarly spaced from shoulder 32 of the mandrel to secure the same action at the sequential period of operation when guide member l i is to be lifted.

It is apparent that the principles of this invention may be modified in many ways without departure from the spirit of this invention. For example, while the drill has been described as employing a dependent shroud member extending below the secondary nozzle of the apparatus it is apparent that a wide variety of shroud or pellet deflecting means may be employed. In this connection it is not necessary that the shroud element extend below the secondary nozzle since the principal requirement of this element is to block upward passage of pellets through the borehole. Consequently, other deflector means may be employed for this purpose. Again, while it has been suggested that the shroud is preferably divided into four segments, any number of segments greater than two may be employed. With three segments, for example, retraction slots in each of three grooves of the mandrel will be maintained at three diiferent levels.

If it is desired to rotate the drill illustrated, 2. lug may be positioned in the upper annular groove of the drill collar against which one of the latching dogs may bear. This will serve to lock the drill with respect to the drill collar so that the entire apparatus may be rotated if desired. The latching arrangement employed to cause sequential lifting of the guide elements as desired may be desirably complemented by a pin and slot arrangement positioned in the mandrel and each guide element to limit movement of the guide elements in the slots of the mandrel. Thus, for example, a pin may be fixed in each of the guide members extending into an elongated slot provided in the bed of the groove in which the guide member slides. This elongated slot will have a length equal to the desired slidable movement of each guide member with respect to the mandrel. Such a slot and pin ar- 10 rangement may also be employed to positively hold each guide member in the slots when the drill apparatus is completely removed from the casing. These and other modifications of the invention are specifically contemplated for use therewith.

What is claimed is:

1. A retractable drill bit comprising in combination: a tubular drill collar having at least one interior circumferential groove, a tubular mandrel element fitting within said drill collar in sliding relationship therewith, said mandrel having at least two exterior circumferentially spaced longitudinal grooves, a plurality of support members each positioned within one of said longitudinal grooves in slidable relationship between said mandrel and said drill collar, a pluralit of shroud segments, one suspended from each of said support members, a latching means pivotally fixed to each of said support members and adapted to extend into one of said circumferential grooves whereby to limit downward movement of each support with respect to the drill collar, and at least one nozzle element fixed to the lower termination of said tubular element.

2. Drill bit as defined by claim 1 including a latching means pivotally fixed to said mandrel and adapted to extend into one of said circumierential grooves whereby to limit upward movement of said mandrel relative to said drill collar.

3. Drill bit as defined by claim 1 including a packing element between said drill collar and said mandrel whereby to provide a fluid seal therebetween.

4. Drill bit as defined by claim 1 wherein each of said longitudinal grooves is provided with an inwardly extending recess at a selected point along its length to receive one of said latching members whereby to transfer lifting force from said mandrel to each of said support members.

5. Drill bit as defined by claim 4 wherein individual recesses are spaced at different vertical levels whereby upon raising said mandrel said shroud segments will be retracted into said drill collar in sequence.

6. Drill bit as defined by claim 1 wherein the lower termination of said mandrel is adapted to bear against each of said shroud elements when said latching means extends into said circumferential groove whereby to wedge said shroud elements against the lower termination of the drill collar.

7. Drill bit as defined by claim 1 wherein four support members, four shroud segments, and four longitudinal grooves are employed, each of said longitudinal grooves having an inwardly extending recess at a selected point along its length to receive one of said latching members whereby to transfer lifting force from said mandrel to each of said support members, two of said recesses being at a different level than the remaining two whereby, upon raising said mandrel, said shroud segments will be retracted into said drill collar sequentially in pairs.

References Cited in the file. of this patent UNITED STATES PATENTS Num er Name Date 1,502,851 Gale July 29, 1924 2,208,457 Hurley July 16, 1940 2,233,260 Hawthorne Feb. 25, 1941 2,287,714 Walker June 23, 1942 2,330,083 Sewell Sept. 21, 1943 

